If holding down power for the allotted hardware poweroff time (sorry, not certain of it off the top of my head; I'm wanting to say 90 seconds?) doesn't work, it may be that the device has simply crashed. Basically, you have to wait for the battery to run down, which may take up to three days. Try plugging in some headphones (with the headphones in your ears) to see if you hear the crackle when they go in; if you do, you know the device is still powered. Once it runs down you may be able to charge it as normal and have a working player.